Polylithionite from Vøra has a chemical composition close to the ideal end-member formula KLi₂AlS₄O₁₀F₂. It has the l M polytype structure with a= 5.196(4), b= 8.960(4), c= 10.051(4) Å, ? = 100.29(5)°. The measured density is 2.82(1) g/cm³, the calculated density 2.84 g/cm³. Optical measurements gave ? = 1.556(1) , ?= 1.559(1) , 2V_ª = 43(3)° on one sample and ? = 1.551(1) , ?= 1.555(1) on another. DTA/TGA curves and infrared spectrum are given. The mineral paragenesis of the V øra syenite pegmatite is reported, and data are given that indicate enrichment of Li in the dark mineral constituents. The Gladstone-Dale relationship applied to micas is shown to give better agreement between calculated and measured mean refractive index using k-values from Larsen & Berman ( 1934) than using the values of Mandarino ( 1976) .